The present invention relates to a carrier wave recovery system and, more particularly, to a carrier wave recovery system for a slotted ALOHA system.
In a slotted ALOHA system which belongs to a family of time division multiple access (TDMA) communication systems, time slots each having a predetermined duration are set up beforehand so that multiple stations may individually transmit signals in bursts using such time slots. A burst demodulator adapted to receive the bursts is required to recover on a burst-by-burst basis a reference carrier wave whose frequency is synchronous to a carrier wave component. In such a prior art system, it has been customary to recover a reference carrier wave by means of a phase locked loop which consists of a phase comparator, a low-pass filter, and a voltage controlled oscillator (VCO) which is phase-locked to a carrier component of a burst. In a time slot where a burst is not inputted, the VCO is brought into a self-driven oscillation state since the output of the phase comparator disappears; generally, in this state of the VCO, the frequency stability is not high. This gives rise to a problem that in the self-driven oscillation state the VCO oscillation frequency fluctuates over a substantial range, resulting in a substantial difference between a carrier component frequency and a VCO oscillation frequency, i.e., initial frequency offset when a burst with the carrier component frequency is inputted. Because an initial frequency offset determines a tuning time of a phase locked loop, a great initial frequency offset makes the tuning time disproportionately long and renders the operation of the phase locked loop itself unstable.